Commonly used techniques for controlling switching power converters comprise pulse width modulation (PWM), constant off-time control and COT control. In switching converters with COT control, external compensation network can be omitted, which simplifies the circuit design of the switching converters. Moreover, switching converters with COT control do not require an error amplifier to regulate the output voltage, so the transient response of the switching converters is improved.
FIG. 1 shows a prior step down converter 100 with COT control. The converter 100 comprises: an on-time generating circuit 101 configured to generate an on-time signal having an on-time TON of a switch P1; an off-time generating circuit 102, receiving a reference signal VREF and a feedback signal VFB, configured to generate an off-time signal having an off-time TOFF; a switching signal generating circuit 103, receiving the on-time signal and the off-time signal, operable to generate gate control signals HSDR and LSDR to control the ON and OFF switching of the transistors P1 and N1; a filter circuit, comprising an inductor L and a capacitor C1, configured to obtain a smooth output voltage VOUT; a feedback circuit, comprising resistors R1 and R2 connected in series, configured to obtain a feedback signal VFB.
For a step down converter, the duty cycle D of the converter can be expressed as:
                    D        =                                            T              ON                                                      T                ON                            +                              T                OFF                                              =                                    V              OUT                                      V              IN                                                          (        1        )            Wherein TON is the on-time of the transistor P1, TOFF is the off-time of the transistor P1, and VIN is the input voltage of the converter 100.
According to equation (1), the switching frequency of the converter 100 can be expressed as:
                              F          SW                =                              1                                          T                ON                            +                              T                OFF                                              =                                                    V                OUT                                            V                IN                                      ×                          1                              T                ON                                                                        (        2        )            
According to equation (2), the switching frequency of the converter circuit will be changed if the input voltage VIN or the output voltage VOUT of the converter 100 is changed. The variation of the switching frequency will cause EMI (electromagnetic interference), EMC (electromagnetic compatibility), noise and other multiple problems.